Morbidity and mortality of dialysis patients is significantly improved upon undergoing renal transplantation. Most patients receiving transplants require chronic immunosuppressive therapy that can cause major side effects, including life threatening infections. Organ tolerance can have major clinical benefits as it eliminates the need for chronic immunosuppression. The long term goal of this project is to understand mechanisms of allograft tolerance, and to study methods of enhancing them. In this study, renal transplants conducted across MHC class I and II barriers will be studied for tolerance induction in a miniature swine large animal model. It is hypothesized that specific regulatory T cells play an important role in allograft tolerance. Animals with long-term tolerance will be studied for isolation and characterization of regulatory T cells using cellular and molecular assays. The availability of histocompatible large animals for the first time enables the in vivo study of regulatory T cells via adoptive transfer experiments. Regulatory cells isolated from tolerant animals will be transferred into naive, histocompatible animals receiving donor specific transplants without immunosuppression. The ability of the regulatory cells to induce tolerance in these animals will be studied.